Recent development of so-called digital color copying machines which separate a color document into colors, electrically read the document, print the obtained color image data on a sheet, and thus copy the color image is outstanding. With the popularization of digital color copying systems, demands have arisen for higher printing qualities of color images.
Particularly, a demand for printing black characters and thin black lines more black and shaper is strong. When a black document is separated into colors, yellow, magenta, cyan, and black color signals are generated as signals which reproduce black, and data is printed based on the obtained color signals. Black is reproduced by superposing the four colors, and thus even slight misregistration between the colors smears a thin black line. An originally black image cannot look black or blurs, greatly degrading the printing quality.
To increase the printing quality, there is proposed a method of, e.g., extracting color information in an image signal or the feature of the special frequency such as a thin line or dot to detect the area of a black character, color character, or the like. Further, the image is separated into areas such as a halftone image area and dot image area, and the respective areas undergo corresponding processes. For example, if the area is a black character portion, the area is monochromated in black.
To meet demands for cost reduction and reduction in power consumption of the light source, so-called contact image sensors (to be referred to as CISs hereinafter) are being utilized more and more. This CIS uses a light source such as an LED or xenon lamp, an equal-magnification imaging optical system such as a SELFOC lens, and a CCD or CMOS line sensor, in addition to an image scanner using a reduction optical system as a combination of CCDs and lenses.
For example, when a document is read by a color copying machine at a reading rate of about 25 ipm (images/min), the reduction optical system using CCDs requires a document surface illuminance of about 30,000 lx. To the contrary, the examination results reveal that the use of the contact image sensor needs only a document surface illuminance of about 3,000 lx which is about 1/10 that of the reduction optical system, and can offer the same S/N in reading at the same reading rate.
As a color CIS which reads 600 dpi at the equal magnification, CISs as shown in FIGS. 10 and 11 are proposed. FIG. 10 shows an example of the sensor pixel array of the color CIS. One 42-μm pixel corresponding to 600 dpi is divided into three in the subscanning direction, and three R, G, and B reading lines are formed parallel to each other at a pixel pitch of ⅓.
There is also proposed an in-line sensor in which one 42-μm pixel is divided into three in the main scanning direction and pixels are aligned in order of R, G, B, R, G, B, . . . , as shown in FIG. 11.
These two sensors, however, suffer the following problems.
The area of the reading aperture is ⅓ the pixel per color, and the sensitivity is low. As a result, the light quantity of a light source must be directly increased, which poses a problem in increasing a speed of 30 sheets/min or more in a copying machine.
The reading position shifts every ⅓ pixel in the subscanning direction or main scanning direction in accordance with R, G, and B color components, and correction processing for alignment must be executed. Assuming that a G component exists at the center, R and B components undergo alignment processing using linear interpolation. Since linear interpolation is smoothing processing, the MTFs of the R and B components become lower by about 30% than that of the G component.
This leads to a blurred image, and the black character determination unit of the color copying machine tends to detect a thin black line as another color owing to the MTF difference between the R, G, and B components of the image. The color copying machine having the black character determination function cannot determine a black character at high precision, and the qualities of black characters and thin lines on a printed image are low.
Especially in the copying machine, the image quality is most important in reading at the equal magnification of 100%, and then important in reading within the range of 70% to 140%. For this reason, the MTF difference between R, G, and B components is undesirable in reading at the equal magnification.
In an existing color 3-line CCD for a reduction optical system, the interval between R, G, and B reading lines is an integer multiple of one pixel such as a pitch of four or two pixels. In reading at the equal magnification, read data of respective colors are stored in a line memory and read out with a shift corresponding to the line interval. Only this processing can realize reading position correction processing.
A larger line interval, however, causes large color misregistration due to mechanical vibrations. Similar to the MTF difference, a thin black line is colored and leads to erroneous black color determination. Hence, the line interval is ideally one line. This also applies to a color CIS.
A general color CCD for a reduction optical system has R, G, and B reading aperture lines formed parallel to the main scanning direction. The color CCD, comprises two analog shift registers for charge transfer/reading in correspondence with each aperture line, i.e., a total of six analog shift registers. These analog shift registers do not pose any problem in the reduction optical system which uses only one color CCD.
In the color CIS, however, 16 color CCD chips are aligned in the main scanning direction. The use of similar color CCDs results in 6×16=96 reading outputs. Wiring and an analog processor circuit for processing the outputs become bulky.
In the pixel structure of the CIS, 468 pixels are arranged on one chip of a CCD reading element for each color. If 16 chips are aligned, 468×16=7,488 pixels are arranged in the main scanning direction. For example, to read an A4-size document in the longitudinal direction at 60 dpi, there is proposed a structure in which the pixel pitch between pixels on each chip in the main scanning direction is decreased at the end, instead of an equal pixel pitch.
A change in pixel pitch does not generate any problem when the CIS having this structure reads a document such as a printing paper sheet having a document image with no periodicity. When a document image such as dotted printed matter with periodicity is read, a change in pitch at the end of the CCD reading element with respect to the spatial frequency of the document image generates moire, and the density changes depending on the phase.
The black character determination unit of a color copying machine adopts an image area separation technique to check whether an area in process is a dotted image or character area. Also in this case, the pixel pitch is desirably equal.